High-resolution, low-latency, bunch-by-bunch feedback system for nanobeam stabilization

We report the design, operation, and performance of a high-resolution, low-latency, bunch-by-bunch feedback system for nanobeam stabilization. The system employs novel, ultralow quality-factor cavity beam position monitors (BPMs), a two-stage analog signal down-mixing system, and a digital signal processing and feedback board incorporating a field-programmable gate array. The field-programmable gate array firmware allows for the real-time integration of up to fifteen samples of the BPM waveforms within a measured latency of 232 ns. We show that this real-time sample integration improves significantly the beam position resolution and, consequently, the feedback performance. The best demonstrated real-time beam position resolution was 19 nm, which, as far as we are aware, is the best real-time resolution achieved in any operating BPM system. The feedback was operated in two complementary modes to stabilize the vertical position of the ultrasmall beam produced at the focal point of the ATF2 beamline at KEK. In single-BPM feedback mode, beam stabilization to <math display="inline"><mn>50</mn><mo>±</mo><mn>5</mn><mtext> </mtext><mtext> </mtext><mi>nm</mi></math> was demonstrated. In two-BPM feedback mode, beam stabilization to <math display="inline"><mn>41</mn><mo>±</mo><mn>4</mn><mtext> </mtext><mtext> </mtext><mi>nm</mi></math> was achieved.